An extracellular adhesion molecule complex patterns dendritic branching and morphogenesis.

Robust dendrite morphogenesis is a critical step in the development of reproducible neural circuits. However, little is known about the extracellular cues that pattern complex dendrite morphologies. In the model nematode Caenorhabditis elegans, the sensory neuron PVD establishes stereotypical, highly branched dendrite morphology. Here, we report the identification of a tripartite ligand-receptor complex of membrane adhesion molecules that is both necessary and sufficient to instruct spatially restricted growth and branching of PVD dendrites. The ligand complex SAX-7/L1CAM and MNR-1 function at defined locations in the surrounding hypodermal tissue, whereas DMA-1 acts as the cognate receptor on PVD. Mutations in this complex lead to dramatic defects in the formation, stabilization, and organization of the dendritic arbor. Ectopic expression of SAX-7 and MNR-1 generates a predictable, unnaturally patterned dendritic tree in a DMA-1-dependent manner. Both in vivo and in vitro experiments indicate that all three molecules are needed for interaction.

Systematic genetic analysis of virulence in the human fungal pathogen Cryptococcus neoformans.

The fungus Cryptococcus neoformans is a leading cause of mortality and morbidity among HIV-infected individuals. We utilized the completed genome sequence and optimized methods for homologous DNA replacement using high-velocity particle bombardment to engineer 1201 gene knockout mutants. We screened this resource in vivo for proliferation in murine lung tissue and in vitro for three well-recognized virulence attributes-polysaccharide capsule formation, melanization, and growth at body temperature. We identified dozens of previously uncharacterized genes that affect these known attributes as well as 40 infectivity mutants without obvious defects in these traits. The latter mutants affect predicted regulatory factors, secreted proteins, and immune-related factors, and represent powerful tools for elucidating novel virulence mechanisms. In particular, we describe a GATA family transcription factor that inhibits phagocytosis by murine macrophages independently of the capsule, indicating a previously unknown mechanism of innate immune modulation.

A link between virulence and homeostatic responses to hypoxia during infection by the human fungal pathogen Cryptococcus neoformans.

Fungal pathogens of humans require molecular oxygen for several essential biochemical reactions, yet virtually nothing is known about how they adapt to the relatively hypoxic environment of infected tissues. We isolated mutants defective in growth under hypoxic conditions, but normal for growth in normoxic conditions, in Cryptococcus neoformans, the most common cause of fungal meningitis. Two regulatory pathways were identified: one homologous to the mammalian sterol-response element binding protein (SREBP) cholesterol biosynthesis regulatory pathway, and the other a two-component-like pathway involving a fungal-specific hybrid histidine kinase family member, Tco1. We show that cleavage of the SREBP precursor homolog Sre1-which is predicted to release its DNA-binding domain from the membrane-occurs in response to hypoxia, and that Sre1 is required for hypoxic induction of genes encoding for oxygen-dependent enzymes involved in ergosterol synthesis. Importantly, mutants in either the SREBP pathway or the Tco1 pathway display defects in their ability to proliferate in host tissues and to cause disease in infected mice, linking for the first time to our knowledge hypoxic adaptation and pathogenesis by a eukaryotic aerobe. SREBP pathway mutants were found to be a hundred times more sensitive than wild-type to fluconazole, a widely used antifungal agent that inhibits ergosterol synthesis, suggesting that inhibitors of SREBP processing could substantially enhance the potency of current therapies.

The transmembrane LRR protein DMA-1 promotes dendrite branching and growth in C. elegans.

Dendrites often adopt complex branched structures. The development and organization of these arbors fundamentally determine the potential input and connectivity of a given neuron. The cell-surface receptors that control dendritic branching remain poorly understood. We found that, in Caenorhabditis elegans, a previously uncharacterized transmembrane protein containing extracellular leucine-rich repeat (LRR) domains, which we named DMA-1 (dendrite-morphogenesis-abnormal), promotes dendrite branching and growth. Sustained expression of dma-1 was found only in the elaborately branched sensory neurons PVD and FLP. Genetic analysis revealed that the loss of dma-1 resulted in much reduced dendritic arbors, whereas overexpression of dma-1 resulted in excessive branching. Forced expression of dma-1 in neurons with simple dendrites was sufficient to promote ectopic branching. Worms lacking dma-1 were defective in sensing harsh touch. DMA-1 is the first transmembrane LRR protein to be implicated in dendritic branching and expands the breadth of roles of LRR receptors in nervous system development.

Exploration of whole-genome responses of the human AIDS-associated yeast pathogen Cryptococcus neoformans var grubii: nitric oxide stress and body temperature.

Cryptococcus neoformans var grubii is an opportunistic basidiomycete yeast pathogen that is a significant cause of HIV/AIDS-related deaths worldwide. We describe a whole-genome oligonucleotide microarray for this pathogen. These arrays have been used to elucidate the transcriptional responses of the genome to heat shock as well as to two conditions relevant to human infections: body temperature and nitric oxide (NO) stress produced by the NO donor DPTA-NONOate. This analysis revealed an NO-inducible C. neoformans-specific four-gene family that showed a highly similar transcriptional profile to that of FHB1, a previously described NO dioxygenase/flavohemoglobin required for virulence. NO treatment also induced genes involved in the synthesis of the antioxidant mannitol, a polyol that accumulates in the cerebrospinal fluid of infected patients. Exposure to NO also caused increased expression of the sole C. neoformans var grubii protein with HHE/hemerythrin cation binding motifs. Notably, a similar gene in E. coli, ytfE, has been shown to be NO-inducible and protects bacterial cells from killing by NO. Genes induced by NO were highly enriched for those repressed at 37 degrees C, indicating an unexpected interplay between temperature and NO regulation in this basidiomycete. Resources described here should facilitate future investigations of this lethal human yeast pathogen.

Parallel beta-helix proteins required for accurate capsule polysaccharide synthesis and virulence in the yeast Cryptococcus neoformans.

The principal capsular polysaccharide of the opportunistic fungal pathogen Cryptococcus neoformans consists of an alpha-1,3-linked mannose backbone decorated with a repeating pattern of glucuronyl and xylosyl side groups. This structure is critical for virulence, yet little is known about how the polymer, called glucuronoxylomannan (GXM), is faithfully synthesized and assembled. We have generated deletions in two genes encoding predicted parallel beta-helix repeat proteins, which we have designated PBX1 and PBX2. Deletion of either gene results in a dry-colony morphology, clumpy cells, and decreased capsule integrity. Two-dimensional nuclear magnetic resonance spectroscopy of purified GXM from the mutants indicated that both the wild-type GXM structure and novel, aberrant linkages were present. Carbohydrate composition and linkage analysis determined that these aberrant structures are correlated with the incorporation of terminal glucose residues that are not found in wild-type capsule polysaccharide. We conclude that Pbx1 and Pbx2 are required for the fidelity of GXM synthesis and may be involved in editing incorrectly added glucose residues. PBX1 and PBX2 knockout mutants showed severely attenuated virulence in a murine inhalation model of cryptococcosis. Unlike acapsular strains, these mutant strains induced delayed symptoms of cryptococcosis, though the infected animals eventually contained the infection and recovered.